The invention relates generally to distillation columns, and more particularly to methods of distillation and distillation columns having a heat pump with a reboiler and an inlet superheater.
The Penex™ process from UOP LLC is designed for the catalytic isomerization of pentane, hexane, and mixtures thereof. As shown in FIG. 1, feed 10 is sent to reactors 15 where reactions take place over a fixed catalyst bed in the presence of hydrogen and at operating conditions that promote isomerization and minimize hydrocracking. The reactor effluent 20 is sent to a product stabilizer 25. The stabilizer bottoms stream 30 can be separated into normal and isoparaffin components by fractionation in a de-isohexanizer column 35. The de-isohexanizer column 35 separates higher octane di-methyl butane C6 isomers and lighter material into an overhead stream 40 and lower octane methyl-pentane C6 isomers and heavier material into a bottoms stream 45. The de-isohexanizer column 35 typically has a side cut where a methyl-pentane rich stream 60 is withdrawn and recycled back to the reactor 10. The overhead stream 40 is condensed in condenser 65, and sent to receiver 70. The receiver outlet stream 75 is divided into a first portion 80 which is returned to the de-isohexanizer column 35 and a second portion 85 which is recovered.
A portion 50 of the bottoms stream 45 is sent to a reboiler 55 where it is heated and returned to the de-isohexanizer column 35. The de-isohexanizer reboiler 55 typically uses low pressure or medium pressure steam as a heat source, and it is one of the largest energy consumers in a naphtha complex. Reducing the steam consumption of the de-isohexanizer column would lower the energy cost of the overall process.
Heat pumps can be utilized economically in hydrocarbon distillation columns where the temperature difference between the overhead and the bottoms stream is low, e.g., less than about 27.7° C. (50° F.). Propane/propylene splitters are examples of columns in which heat pumps can be used economically. Such columns are described in U.S. Pat. Nos. 4,753,667, and 7,842,847, for example. Other examples of the use of heat pumps in separation processes are found in U.S. Pat. Nos. 4,336,046, 4,559,108, and 7,908,861.
However, the use of a heat pump in a column for separating C5 and C6 components is typically not economical because the higher temperature difference between the overhead stream and the bottoms stream (e.g., 38.9° C. (70° F.) or more) requires higher compression ratios in the heat pump compressor and therefore higher compression costs.
Furthermore, compression of the C5 and C6 overhead vapor stream results in partial condensation of the stream. In order to avoid this condensation, the overhead vapor stream must be superheated before it enters the heat pump compressor. Galstaun et al., Heat pumping pays out in C5/C6 isom plant, Oil & Gas Journal, Nov. 12, 1979, pp. 223-226, discusses the use of steam to preheat the vapor before it enters the compressor. The requirement for steam superheating further erodes the economics of the heat pump.